Ion implantation has become a standard accepted technology of industry to dope workpieces such as silicon wafers or glass substrates with impurities in the large scale manufacture of items such as integrated circuits and flat panel displays. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.
Conventional ion sources consist of a chamber, which may be formed from graphite, having an inlet aperture for introducing a gas to be ionized into a plasma and an exit aperture through which the plasma is extracted to form the ion beam. In general, the plasma comprises ions desirable for implantation into a workpiece, as well as ions which are not desirable for implantation and which are a by-product of the ionization process. In addition, the plasma comprises electrons of varying energies.
One example of such an input gas is phosphine (PH.sub.3) which is utilized to produce positively charged phosphorous (P.sup.+) ions for doping the workpiece. The phosphine may be diluted within the source chamber with hydrogen gas, and high energy electrons emitted from an energized filament within the source chamber bombard the mixture. As a result of this ionization process, hydrogen ions are produced which may be extracted through the exit aperture, along with the desired P.sup.+ ions, into the ion beam. Thus, the hydrogen ions will be implanted along with the desired ions. If a sufficient current density of hydrogen ions is present, these ions may cause an unwanted increase in the temperature of the workpiece that may actually damage the photoresist on the surface of the substrate.
In order to reduce the number of unwanted ions available for extraction into the ion beam, it is known to provide magnets within the source chamber to separate the ionized plasma. The magnet confines undesirable ions and high energy electrons to the portion of the source chamber away from the exit aperture and confines the desirable ions and low energy electrons to the portion of the source chamber near the exit aperture. Such a magnet arrangement is shown in U.S. Ser. No. 08/756,970 now U.S. Pat. No. 5,760,405 to the assignee of the present invention, incorporated by reference herein as if fully set forth. Other related examples of magnet configurations within an ion source chamber are shown in U.S. Pat. No. 4,447,732 to Leung et al., and Japanese Patent No. 8-209341 to Haraichi. Both of these references show a magnetic filter comprised of a plurality of longitudinally extending magnets oriented parallel to each other.
In applications for implanting large surface areas, such as flat panel displays, a ribbon beam ion source may be utilized. The ribbon beam is formed using a plurality of elongated exit apertures in the source chamber, as shown in U.S. Ser. No. 08/756,970 now U.S. Pat. No. 5,760,405. The plurality of exit apertures provides the capability for adjusting the width of the ribbon beam, and also provides for greater variability of beam current density and energy than a single aperture would otherwise provide. Each of the plurality of exit apertures outputs a portion of the total ion beam output by the ion source. Beam portions output by apertures located between surrounding apertures overlap the beam portions output by those surrounding apertures.
The use of a magnetic filter such as that shown in U.S. Pat. No. 4,447,732 or Japanese Patent No. 8-209341 in a multiple aperture ribbon beam ion source, however, results in undesirable ion beam current characteristics. Specifically, orientation of the longitudinally extending (columnar) magnets orthogonally with respect to the elongated exit apertures of the ion source results in beam current nonuniformities along the length of the ribbon beam. These current nonuniformities result from regions of increased current, which are output from each aperture nearest, the locations of the magnets. With multiple apertures, and with the orthogonal positioning of the magnets with respect to these apertures, this effect is cumulative for each aperture, resulting in significant variances in total beam current along the length of the ribbon beam. The current non-uniformity can result in non-uniform ion implantation of the workpiece.
Accordingly, it is an object of the present invention to provide a magnetic filter for a ribbon beam ion source, which provides a ribbon ion beam having a uniform current density along the entire length thereof.
It is a further object of the present invention to provide a magnetic filter for an ion source which does not suffer from the undesirable beam current characteristics that are inherent with known ion source magnetic filters.